There has been a demand in recent years for effective utilization of low-grade fuel, such as coal and heavy oil, in view of the diversification that is seeing not only high-grade fossil fuel but also low-grade fossil fuel being actively used. Also, in the field of thermal power generation, from a view point of increasing power generation efficiency, combined cycle power generation in which a gas turbine using a gas fuel and a steam turbine are both used and power generation in which hydrocarbon gas is introduced to a fuel cell have come into wide use. In such a situation, research and development have been carried out to utilize a gasified low-grade fuel for power generation.
Since low-grade fuel generally contains a large amount of sulfur compounds, if it is gasified and burned without being subjected to treatment, the sulfur compounds are exhausted as sulfur oxides into the air through a stack, which causes environmental degradation such as acid rain. Therefore, in ordinary thermal power generation, what is done in practice is to provide an exhaust gas desulfurizer on the downstream side of a boiler for removing the sulfur compounds, for example, as gypsum. However, in the combined cycle power generation, materials are markedly corroded because the temperature at the inlet of gas turbine is higher than the temperature of boiler in the ordinary thermal power generation. To solve this problem, it is necessary to protect the materials by removing various kinds of impurities including sulfur compounds on the upstream side, not on the downstream side, of the gas turbine, which means that the aforementioned exhaust gas desulfurizer cannot be used. In fuel cell power generation as well, the securing of power generation efficiency and durability for the protection of materials is a must, and it is necessary to remove various kinds of impurities on the upstream side of the fuel cell.
As a method for removing the impurities, what is called a wet-type gas purifying process, in which water-soluble components are removed with a water scrubber and H2S (hydrogen sulfide) is removed with aqueous solution of amines, has been used in actual practice. However, the aqueous solution of amines is incapable of removing COS, although it is capable of removing H2S. Therefore, hydrolysis expressed by Formula (1) is carried out using a COS conversion catalyst to accelerate a reaction in which COS is converted to a form of H2S that can be removed with the aqueous solution of amines.COS+H2O→H2S+CO2  (1)
As a COS conversion catalyst, a catalyst containing titania (refer to Japanese Patent No. 1463827, Japanese Patent Provisional Publication No. 11-80760(No. 80760/1999), etc.), a catalyst containing alumina, a group IV metal, and barium, and a catalyst containing an alkali metal, chromium oxide, and alumina are known (Japanese Patent Provisional Publication No. 2000-248286). However, these catalysts have a problem in that catalytic activity is decreased by unburned O2 that is present in minute quantities (in the order of ppm) in the gasified gas. Also, since nitrogen that is introduced from the outside to provide a seal also contains a minute quantity of O2, the problem of decreased catalytic activity becomes more serious.
In order to prevent the decrease in activity of the COS conversion catalyst due to the O2, a process in which a combustion catalyst is provided on the upstream side of the COS conversion catalyst has also been devised. However, this process has some problems in that the combustion catalyst is expensive because it generally carries a noble metal, the combustion catalyst is prone to be poisoned by H2S and thus cause a decrease in performance, and heat generation on the combustion catalyst gives great stress to the peripheral equipment.